1. Field of the Invention
The present invention relates to a thermal printer which keeps a print density at a constant level by controlling a drive pulse, and more particularly to a thermal printer which determines an optimum driving time period, e.g. voltage application time period or energization time period, of a print head in accordance with the number of dots to be simultaneously heated and based on the detected voltage, in order to keep constant the print density irrespective of a change of the voltage applied to the thermal head due to a change of the number of dots to be simultaneously heated.
2. Description of the Prior Art
The print density in the thermal printer is determined by a calorific quantity of dots. In order to attain a uniform print density, it is necessary that a calorific quantity per dot of a thermal head be constant. That is, the calorific quantity per dot given by EQU W=v.sup.2 t/R (1.1)
where
W (mf) is a calorific quantity per dot, PA1 R (.OMEGA.) is a resistance per dot, PA1 V (volt) is an applied voltage per dot, and PA1 t (ms) is a heating time,
is constant. For example, when the resistance R per dot is 11 .OMEGA. and the calorific quantity W per dot is 2.1 (mJ), a relation between the applied voltage to the thermal head and the heating time is determined from the equation (1.1) as shown in FIG. 1.
When a dry battery is used as a power supply, the applied voltage to the thermal head decreases as an electromotive force of the dry battery decreases. As a result, the print density for a given heating time is reduced. In the prior art, the print density is kept at a constant level by increasing the heating time as the electromotive force of the dry battery decreases.
For example, a dummy load for a print operation (e.g. a load when the number of simultaneously heated dots is one) is connected to the dry battery and a voltage across the battery is detected to determine an optimum heating time for the detected voltage.
In a thermal printer which uses a four-phase pulse motor as a stepping motor, resistances of two phases of the motor and a resistance determined by the number of dots to be simultaneously heated constitute the load. Table 1 which shows the resistances when the printer is driven shows a variation of the resistances of the printer due to the number of dots to be simultaneously heated.
TABLE 1 ______________________________________ Printer Printer Operation Resistance R ______________________________________ Voltage Detection (4-phase excitation) 7.5.OMEGA. 1-dot print (2-phase excitation + 1 dot) 6.3.OMEGA. 2-dot print (2-phase excitation + 2 dots) 4.0.OMEGA. 3-dot print (2-phase excitation + 3 dots) 2.9.OMEGA. 4-dot print (2-phase excitation + 4 dots) 2.3.OMEGA. 5-dot print (2-phase excitation + 5 dots) 1.9.OMEGA. 6-dot print (2-phase excitation + 6 dots) 1.6.OMEGA. 7-dot print (2-phase excitation + 7 dots) 1.4.OMEGA. Remarks 4-phase pulse motor winding resistance: 30.OMEGA./phase 1 .times. 7 thermal head: 11.OMEGA./dot ______________________________________
When the dry battery is used as the power supply, the applied voltage to the thermal head also changes as shown in FIG. 2 because a ratio of an internal resistance of the dry battery and the load resistance changes.
As seen from FIG. 2, the applied voltage to the thermal head significantly changes depending on the number of dots to be simultaneously heated.
In the prior art, however, the applied voltage to the thermal head is determined by connecting the dummy load, thereafter the heating time is determined. Accordingly, the heating time is determined independent of the number of dots to be simultaneously heated, resulting in a variation in the print density. If a plurality of dots are printed with the same heating time as that which is the maximum for the applied voltage to the thermal head when heating one dot, the print density is reduced because the applied voltage to the thermal head when the plurality of dots are heated is lower than that when one dot is heated.